(a) Field of the invention
The present invention relates to a large aperture long focus macro lens system.
(b) Description of the prior art
In case of lens systems for photographic cameras in general, aberrations vary when the object distance is varied. Therefore, the range of photographable object distance is limited. When the infinite object distance is included, the range of photographable object distance of known lens systems for photographic cameras is generally limited to about 1/10 as the photographing magnification. Some of known lens systems for photographic cameras are arranged to be capable of photographing up to the photographing magnification of 1/2 or 1 by using a so-called aberration correcting mechanism for short distance photographing (a mechanism for correction of aberrations when focusing on an object at a short distance) which is arranged to move some of lens components in order to correct the variation of aberrations to be caused at the time of focusing. However, said known lens systems have problems as shown under (a) to (g) below.
(a) The aperture ratio is small, i.e., about F/2.8 at the best. PA1 (b) The telephoto ratio is large, i.e., larger than 1.4 in some cases, and the overall length of the lens system is long. PA1 (c) The lens advancing amount for correction of aberrations at the time of focusing is extremely large, and this is not desirable from the view points of the structure of the lens mount and operability. PA1 (d) The number of lenses constituting the lens system is large and, in some cases, it exceeds 10. PA1 (e) In cases of some of said known lens systems, aberrations are not corrected satisfactorily favourably when focused on an object at a short distance. PA1 (f) In cases of some of said known lens systems, the photographing magnification does not become higher than 1/5 though a very effective aberration correcting mechanism is provided. PA1 (g) In cases of some of said known lens systems, the focal length of the lens system becomes short when the lenses are advanced for the purpose of focusing, and the working distance becomes short at the time of a high magnification. PA1 (1) -0.4.phi.&lt;.phi..sub.III &lt;0.4.phi. PA1 (2) 0.33&lt;(.DELTA..sub.12 -.DELTA..sub.3)/.DELTA..sub.12 &lt;0.85(.DELTA..sub.3 &gt;0) PA1 (3) 0.70.phi.&lt;.phi..sub.A &lt;1.8.phi. PA1 (4) 0.60.phi.&lt;-.phi..sub.B &lt;1.7 PA1 (5) n.sub.A &gt;n.sub.B PA1 (6) 0.6.phi.&lt;1/r.sub.5 &lt;1.5.phi. PA1 (7) 0&lt;.vertline.1/r.sub.9 .vertline.&lt;1.3.phi. PA1 (8) 0.1f&lt;d.sub.3 &lt;1.17 PA1 (9) 0.14f&lt;.SIGMA.D.sub.III &lt;0.23f PA1 (10) 1.7&lt;n.sub.6
It is considered that the causes of the above-mentioned problems are as explained below.
The problem (a) is caused due to the fact that it is difficult to correct the variation of spherical aberration and coma which occurs when the distance to the object (photographing magnification) is varied, and it is unavoidable to make the aperture ratio small (the F-number large) is order to cut off the portion where the variation of said aberrations is large.
The problem (b) occurs because it is advantageous for making the variation of aberrations small when refractive powers of respective lens groups are made small and, as a result, the telephoto ratio becomes large.
The problem (c) occurs because the variation of aberrations to be caused when focusing on an object at a short distance lens groups each having refractive power of a certain degree.
The problem (d) occurs because it is more advantageous for correction of aberrations when the number of lenses constituting the lens system is larger.
The problem (e) occurs owing to inadequate selection of lens configuraiton of the lens system, inadequate selection of a lens component to be moved for correction of aberrations when focusing on an object at a short distance, and inadequate amount of movement of said lens component.
The problem (f) occurs because the lens configuration of said lens component to be moved is not adequate.
The problem (g) occurs when the lens system consists of a front lens group having positive refractive power and a rear lens group having negative refractive power.
Regarding the above-mentioned problems, actual examples of lens systems are discussed below based on the known lens systems disclosed respectively in Japanese published unexamined patent applications Nos. 28038/80, 107210/81, 192916/82, 21221/83, 152414/84, etc.
Out of said known lens systems, the lens system disclosed in Japanese published unexamined patent application No. 28038/80 has the problems (a), (b), (c) and (e). The lens system disclosed in Japanese published unexamined patent application No. 107210/81 has very excellent performance and is capable of photographing up to the photographing magnification about 1. However, it has the problems (a), (b), (e) and (g). In case of the lens system disclosed in Japanese published unexamined patent application No. 192916/82, the problem (a) is solved satisfactorily. However, said known lens system has the problems (b), (c) and (e). Besides, there are two airspaces to be varied at the time of focusing and, therefore, the structure of the lens mount is complicated. The lens system disclosed in Japanese published unexamined patent application No. 21221/83 has the problem (f). The lens system disclosed in Japanese published unexamined patent application No. 152414/84 has the problems (b), (c) and (g) which should be solved. Out of them, the problems (b) and (c) are considerably detrimental to a semi-telephoto lens system with the focal length about 90 mm to 100 mm from the view points of the structure of the lens mount, size of the lens system and operability. As for the problem (g), the focal length of the lens system becomes short when focused on an object at a very short distance. As a result, the working distance becomes short or the perspective effect is lost at that time, and this is contrary to the object to be attained by arranging the lens system as a semi-telephoto lens system.